Drugs of abuse bypass the normal mechanisms that regulate release of dopamine, producing longterm changes in the neural systems that govern reward. Association with dopamine release enables environmental cues to activate the reward pathway, thus promoting either adaptive behavior or drug addiction. However, we still understand little about the physiological mechanisms that regulate dopamine release, the changes in release that underlie both normal, adaptive behavior and addiction, or the mechanisms by which dopamine controls the reward pathway. Dopamine differs from other classical transmitters in several respects. First, dopamine activates G protein-coupled receptors through volume transmission, making it difficult to understand how dopamine can convey a signal with the temporal resolution required to associate transient environmental cues with reward. Interestingly, recent work has shown that dopamine neurons also release glutamate, but the physiological role of glutamate co-release remains unclear. Second, dopamine undergoes release from dendrites as well as presynaptic boutons, and dendritic dopamine release has been considered to play a particularly important role in plasticity of the reward system. Dendritically released dopamine activates autoreceptors on dopamine neurons, and through Dl receptors, appears to influence synaptic inputs onto midbrain dopamine neurons and to control outflow from the basal ganglia through the substantia nigra pars reticulata (SNr). However, the mechanisms responsible for dendritic dopamine release and the physiological role of Dl receptor activation in the midbrain remain unclear. This program will thus address multiple presynaptic mechanisms highly relevant to the release and action of dopamine in the reward pathway: 1) The mechanism of dendritic dopamine release (Projects 1 and 3); 2) The mechanism and physiological role of glutamate co-release by dopamine neurons (Project 3); 3) Trafficking and regulation